There are requirements in large countries throughout the world to provide high audio quality, multiple program radio broadcasts. Historically radio broadcasting started with terrestrial stations using low transmission frequencies with AM (Amplitude Modulation). Due to its analog narrowband characteristics, susceptibility to interference and propagation variability, AM terrestrial stations cannot transmit high audio quality broadcasts nor, beyond a local area, provide reliable wide geographical coverage. Subsequently, terrestrial radio stations using higher transmission frequencies with FM (Frequency Modulation) were implemented. These transmit much better quality audio programs, but each terrestrial FM station provides only local geographical coverage (e.g., a 40 km radius is typical) and the signal is analog. Currently several systems which could provide high quality (e.g., compact disc) terrestrial radio broadcast on a local geographical basis using digital modulation are under technical evaluation (Broadcast Engineering Conference Proceedings; National Association of Broadcasters; April., 1996; pp. 5-26 and 115-125).
The requirement to provide high quality, multi-program radio broadcasting on a broad geographical basis (e.g., nationwide, regional or hemispherical) through use of satellites to transmit the radio programs directly to fixed, mobile and transportable receivers on the earth has been proposed (International Journal of Satellite Communications; Vol. 13, Nr. 4; July-August 1995; pp. 215-272). With the current state of art and projected advancements in satellite technology, such radio broadcasts from satellites must be within direct line of sight of the user receivers, although multiple satellites can be used to mitigate blockage and certain partial blockage, such as tree foliage and multipath. Multipath can also be mitigated by using higher satellite transmission power. However, current and projected satellites do not have the power transmission capability to penetrate modern buildings or to provide service where heavy satellite signal shadowing exists, such as in the central cores of large cities, with reliable reception of high quality multi-program audio programs. It has been estimated that such blocked or heavily shadowed transmissions would have to be at least a thousand times more powerful than a non-blocked satellite transmission (Direct Broadcast Satellite Radio; Nasser Golshan; California Institute of Technology (JPL D-9550); March 1992; pp. 13-18) and, even if this could be eventually accomplished, the cost of the extremely high powered satellites and their launch vehicles would make the service uneconomical.
A solution for providing high quality, multi-program satellite radio broadcast service available to listeners in fixed locations, such as homes and office buildings, and in automobiles moving through heavily shadowed areas is to broadcast the radio service in these particular areas of the country by associated terrestrial broadcast radio stations. This can be accomplished by having such terrestrial broadcast stations receive the satellite transmission and rebroadcast it in a different radio frequency band. Generally there are no suitable additional frequency bands available to accomplish the rebroadcast, and all mobile radio receivers would be more expensive due to the need to receive two frequency bands. It is therefore important to devise methods where the satellites and associated terrestrial broadcast stations use the same transmission frequency without interfering with each other.
A method to use the same radio frequencies for a combined satellite broadcast and terrestrial rebroadcast system has been proposed by French and Canadian organizations (Digital Audio Broadcasting Proceedings; European Broadcasting Union; June, 1992; pp. 99-109) and experimental results reported (Digital Audio Broadcasting Proceedings; Canadian Association of Broadcasters; March, 1994; pp. 206-235). The method uses a modulation called COFDM (Coded Orthogonal Frequency Division Multiplex), and it has been shown theoretically and by measurement (Digital Audio Broadcasting Proceedings; Canadian Association of Broadcasters; March, 1994; p. 179) that coverage gaps occur within the service area when the satellite and terrestrial signals have similar amplitudes but arrive at user receivers with a time differential greater than the system's allowable guard band interval.
The methods and apparatus of this invention use spread spectrum modulation configured to allow satellite radio broadcasting and associated terrestrial radio rebroadcasting of high quality, multi-program audio programs at the same radio frequency so that user reception is possible at all locations ubiquitously over very large geographical service areas.
In part, this invention relates to the inventions disclosed in U.S. Pat. Nos. 5,278,863, issued on Jan. 11, 1994, and 5,319,673, issued on Jun. 7, 1994, and U.S. patent application Ser. No. 08/434,228, filed in the U.S. Patent & Trademark Office on May 4, 1995. By these references, these disclosures are incorporated by reference in this disclosure.